These studies will investigate the effects of post-natal age on pulmonary vascular reactivity, lung fluid balance, and endothelial modulation. Particular emphasis will be placed on the interactions with eicosanoids, as they are important for the normal transition of pulmonary hemodynamics which occurs at birth. Responses of the developing pulmonary circulation to eicosanoids will be evaluated at three ages: 0-4 days, 12-16 days, 2-3 months for lambs and .3 months (adults) for guinea pigs. The responses to be determined included magnitude and site of vascular reactivity to eicosanoids, their effects on neonatal lung fluid balance, and the potential interaction between age-related vascular reactivity to eicosanoids and modulation by the pulmonary vascular endothelium. The first specific aim of this proposal is to determine the site of pulmonary vascular responses to eicosanoids as a function of age in isolated perfused lamb lungs. These experiments will elucidate which vessels are most responsive to vasoactive eicosanoids, and how their reactivity is altered by development changes. The responses will be determined during normoxia and during hypoxia, as there is a significant interaction of 02 tension and pulmonary vascular tone an d/or reactivity. The second specific aim is to evaluate the effects of eicosanoids on lung fluid balance during this developmental period in isolated perfused lamb lungs. These studies will attempt to determine the contributions of hydrostatic pressures, vascular surface area, and vascular permeability with eicosanoid-induced pulmonary edema. The final specific aim will be to determine whether there is a difference in the modulation of pulmonary vascular reactivity by the endothelium due to developmental changes. Using isolated intra-pulmonary vessels from guinea pigs, the first goal os to determine the responses to classic endothelium-dependent and -independent compounds as a function of age. The second of this specific aim will be to determine the interaction of the endothelium with eicosanoids.